Slope Failure and Landslide Mechanism, Murree Area
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Geological Bulletin Univ. Peshawar Vol. 35, pp. 125-137,2002 IFTIKHAR AHMED ABBASI', M. ASIF KHAN^, MOHAMMAD ISHFA' & P.K. ~00~4 'Department of Geology, University of Peshawar 'NCE in Geology, University of Peshawar 3LandmarkResources, Islamabad 41CIMOD,Katmandu, Nepal. ABSTRACT: The area around Murree town is inherently vulnerable to mass movement p rocesses, most commonly due to landsliding. It is characterized by high degree of erosivity and erodibility due to both natural and man inducedfactors. The area ispart of ayoung orogenic belt that is experiencing one of the world's most rapid upliff rates, and is located in the near vicinity of major thrustfaults. Shear zones associated with still activefaults, such as Murree Thrust and also thefoldinggenerated due to thrustinginduce an inherited weakness and natural instability in the rocks of the area. This compounded with ever increasingpopulation pressure, deforestation, large-scale construction work in theform of residential and commercial buildings and road network havefurther aggravated the problem. Lithologically the area is comprised of shale, siltstone and sandstone belonging to the Oligocene-Miocene Murree and Kuldana formations. In order to study the landslides and other mass movement problems in Mun-ee, an area around the main town and alongtheMuwee-Kashmir Highway is selectedfor detailed analysis of theproblem. The eastern slopes of the Murree ridge, and the road section between Jhika Gali and Aliot village are worst affected by the landsliding. Around Murree town, major landslidessuch as Shijang Hotel, Midway and MIT landslides, located on the upslopes of the Jhika Gali-Lawrence College Bypass road are caused by inadequate drainagesystemfrom the city centre (Murree ridge). In mostpart of the area around Murree ridge, creep is a common phenomenon in thick colluvium coverdue to obvious loss of vegetation. The area between Jhika Gali and Aliot village is aZso afected by landsliding, and some of the largest landslides such as Aliot, Birgran and Kasseri landslides have inflictedgreat damage tofarm andforest land. This area is located in thefootwall of the MBTand is deformed byfolding and termination splays associated with it. Mast of the landslides initiated at the colluvium-bedrock interface, but once initiated the tension cracks spread into the bedrock and therefore causing thefailure in it. Major landslides in the area exhibita complex array ofjointflracture pattern both parallel and perpendicular to the bedding, and in various ways contribute to the mass movement. d INTRODUCTION collision of the Indian plate with the Eurasian plate during Eocene time along a suture zone known as The town ofMurree in Himalayan foothills lies the Main Mantle Thrust (MMT) (Tahi&heli et al,, about 50 km to the northeast of Islamabad, the 1979; Coward et ale, 1986), and is one of the ~pitalof Pakistan (Fig. I), at an elevation of approxi- world's most rapidly uplifting region on earth (Zeitler, mately 2200m. The Himalaya was created by the 1985). The deformation progressively migrated Geological control on natural hazards: earthquakes and mass movement. Khan, M.A., Abbasi, LA., & Khattak, G.A. (Eds.). Geological Bulletin, University of Peshawar, (Special Issue), Vol. 35, pp. 125-137, 2002 Geological Bulletin Univ. Peshawar Vol. 35, py. 125-137,2002 126 southward away from the Early Eocene collision nism of failure in the landslide prone parts of the zone at the MMT and reached the Himalayan foot- area, The study was conducted with the help of Survey hills by Miocene time. The Himalayan foothills, ofPakistan topogaphic maps, interpretationof digital commonly known as the sub-Himalayas (Gansser, SPOT panchromatic image, and extensive fieldwork. 1964) are defined as a set ofrocks which are bounded by major thrust faults such as Main Boundary STRUCTURAL GEOLOGY Thrust (MBT) and the Main Frontal Thrust (MFT) Structurally the area around Murree is com- to the north and south respectively (Fig. 1). The plexly deformed by MBT and folding associated with Murree area lies in a seismically active zone due it (Fig. 2) (Chambers, 1992; Iqbal and Bannert, to its proximity with the active Murree Thrust 1998). The Murree Thrust trending NE-SW, is an (local name for MBT) along which magnitude 4.5 to emergent fault passing close to Bansra Gali, Chitta 5.0 earthquakes on the Richter scale have been Mor, Sunny Bank and Kuldana (Fig. 2). Most part of recorded as recently as 1977. Nevertheless, earth- the central ridge on which the Murree Town is quakes of magnitude 6 and more can be expected located is a SW plunging syncline with the Murree along this zone. Formation lying in the core ofthe syncline, and the The molasse sediments (siltstone and sand- beds dipping inwards towards the core ofthe ridge. stone) of the Murree and Kuldana formations (Oli- This structure provides a degree ofstructural stabil- gocene-Miocene) constitute most part ofthe Murree ity to the ridge. At the same time, combined with a Hills. The sandstone of the Murree Formation is red lithological setup of alternating sandstone-siltstone- to reddish gray in colour, multistoried, fine to claystone layers, this structure makes the Murree medium grained, cross-bedded, hard and compact ridge a major reservoir ofwater. This is evident from constitutes 4-6m thick sequences that occasionally the presence ofmany springs and seepages at the toes attain thickness of over 10m. The sandstone se- of major sandstone horizons (Rafiq et al., 1989). quences are interbedded with red colour, 10's of This seepage often facilitates slippage ofthe rock and meters thick siltstone and shale. These fine-grained soil bodies at slopes and thus contributes to mass facies constitute most part of the formation and movement. provide a week zone for most of the faults to pro- The topographichigh ofthe Pindi Point is in the pagate through it, Landsliding and other mass move- core of a major anticline in which over 2km thick ment phenomenon are inherited to the area due to section of the Murree Formation molasse is exposed. weak component lithologies, complicated and in- The bedding youngs towards north and dips range tense deformation compounded in recent times by from 20-60" to almost vertical across the fold axis. anthropogenicfactors like deforestation, construction Near Bansra Gali steeply dipping strata becomes and over-population etc (Neiderer and Schafber, 1989). older, transitionally passing from the Kuldana For- This paper describes landslide prone areas mation into the Chor Gali Formation (late Eocene around the Murree town, especiallyalong major road age), characterized by platy, nodular, nummulitic sections, i.e., Bansra Gali-pika Gali-LawrenceCol- limestone and interbedded greenish marls. In this lege ring road, Jhika Gali-Aliot, and the Jhika Gali- area the Murree Thrust is SE verging and brings the Bhurban road sections along the Kashmir Highway Chore Gali Formation over the Murree Formation (Fig. 2). 1t describes some selectedlandslides from indicating a mainSE transport direction along the the area with special emphasis on the geological Murree Thrust. Kashmir point lies tothe southeast processes that contributed in the initiation of the of the axis of the major syncline with NW dipping mass movement processes. Detailed geotechnical (50-60") outcrops. The area between Kashmir Point analyses were conducted tounderstand the mecha- and Jhika Gali appears to have been folded by a series Geological Bulletin Univ. Peshawar Vol. 35, py. 125-137,2002 A / Main KarakuramThrust Gilgit e Kohistan Island Arc 100 km Major Thrust Faults River Features T -*--- + Federal Boundary N Fig. 1. Generalized map of N. Pakistan with study area around Murree as inset. Geological Bulletin Univ. Peshawar Val. 35, pp. 125-137,2002 Kls Als Mls CIS Shls Mwls Ksls Dls Sb MR. AR Bg Kd Bb Gc Cm + -E J- Fig. 2. Detailed location and landslide distribution map of Murree town and surrounding area. Most of important landslides especially those along the Bansragali-Jikagali-Lawrence College ring road, and JikagaIi-Bhurban, JikagaliAliot road sections are marked. Main Boundary Thrust (MBT) is oriented NE-SW direction. Localized faults are marked as broken lines. of isoclinal folds. A large number of inter-molasse planes) behind the slope face is a determining factor faults may be present in this area but are difficult to for slope instability. In most of the studied area, recognize in the field. The shear zones associated particularly in major landslide areas such as Kashmiri with still active Murree Thrust and also the folding Bazar and ChittaMor landslides a detailed survey of generated due to thrusting induce an inherited weak- the joint orientation was carried out, ness and natural instability potential in the strata of the Murree hills. These are potentially weakzones in LANDSLIDE PROCESSES AND a slope and their strength is less than the surround- MECHANISM ing intact rock. Presence of smallshear zones in shale In order to understand the processes and mecha- beds further reduces its strength that is crucial for its nism involved in the landslide activity in the area, stability. Deformation due to faulting and orienta- two major landslides are selected for detailed stud- tion of rock discontinuities (joints and bedding ies, one at Chitta Mor and other at Kashmiri Bazar Geological Bulletin Univ. Peshawar Vol. 35, pp. 125- 137,2002 (Fig. 2). These slides are mapped at 1:500 on Field observations topographic maps prepared with the help of theodo- The landslide is typically a translational type lites and plane tabling. with a complex nature of displacement (Figs. 3). The KASHMIRI BAZAR LANDSLIDE escarpment of the slide is clear on the hillside face and is irregular in shape (Fig. 3). The maximum width This landslide is located about 5 km NE of of the unstable area measures to be about 100 meters Murree, on the Murree-Kashmir road (Fig. 2). Locally along the road section, but is substantially reduced this road connects Murree with the prime hotel to about 20m in the downslope ofthe road (Fig.